Interface circuit for connecting a microphone circuit to a preamplifier

ABSTRACT

An interface circuit is provided that is adapted to connect a microphone circuit to a preamplifier. The microphone circuit has a microphone and at least an output node and the preamplifier has at least an input node connected to the output node by the interface circuit. The interface circuit has at least a decoupling capacitor for DC decoupling the input node from the output node. The decoupling is connected between the input node and the output node. The interface circuit has at least one active circuit, comprising a resistor connected the decoupling capacitor. The resistor acts as part of a resistance multiplier and has an equivalent resistance that together with the decoupling capacitor defines a high-pass filter connected between the microphone and the preamplifier. The interface circuit may also have a biasing circuit connected to the resistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase application submitted under 35U.S.C. §371 of Patent Cooperation Treaty application serial no.PCT/EP2011/068029, filed Oct. 14, 2011, and entitled INTERFACE CIRCUITFOR CONNECTING A MICROPHONE CIRCUIT TO A PREAMPLIFIER, which applicationclaims priority to European patent application serial no. EP 10191206.1,filed Nov. 15, 2010, and entitled INTERFACE CIRCUIT FOR CONNECTING AMICROPHONE CIRCUIT TO A PREAMPLIFIER, and to U.S. provisionalapplication Ser. No. 61/429,930, filed Jan. 5, 2011, and entitledINTERFACE CIRCUIT FOR CONNECTING A MICROPHONE CIRCUIT TO A PREAMPLIFIER.

Patent Cooperation Treaty application serial no. PCT/EP2011/068029,published as WO 2012/065793, and European patent application serial no.EP 10191206.1, and U.S. provisional application Ser. No. 61/429,930, areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an interface circuit for connecting amicrophone circuit to a preamplifier.

BACKGROUND

One of the main aspects to take into account in the development of anelectronic device, especially mobile devices, is the area occupation. Inthe field of mobile devices, such as mobile phones, the reduction of thearea occupation on the Printed Circuit Board (PCB) is a key point inorder to create phones with much more functionalities without alteringtheir dimensions. The focus, during the years, has been to integrateinside a chip, where possible, all those passive components such asresistors, capacitors and inductors which represent the main limit forthe area reduction. Inside old generation phones, such components wereSMD (Surface Mount Devices) mounted directly on the main board. Lateron, thanks to improvement in the technology, these devices were placedinside the chip package, a methodology known as PDI (Passive DeviceIntegration), and in some cases directly integrated inside the chip.However, when it comes to the microphone preamplifying path theimplementation of this approach has not been possible due to the hugecapacitance value of the decoupling capacitors needed between themicrophone and the preamplifier.

FIGS. 1 and 2 show two known ways, single-ended and differential,respectively, to bias and connect the microphone circuit MC_(S), MC_(D)to a preamplifier PA_(S), PA_(D) using an RC network. The microphonecircuit MC_(S), MC_(D) comprises a microphone 3 and a biasing circuitR_(MB1), R_(MB2), R_(MB3), C1, C2, feed by a bias voltage V_(BIAS).

The DC bias voltage of the signal coming from the microphone circuitMC_(S), MC_(D) at the output nodes M_(O), M_(O)′ of the microphonecircuit MC_(S), MC_(D) will depend exclusively by the biasing circuitR_(MB1), R_(MB2), R_(MB3), C1, C2 and is usually different from the DCbias input voltage of the preamplifier PA_(S), PA_(D). The levelshifting between the microphone 3 and the preamplifier PA_(S), PA_(D) DCbiasing voltages is commonly obtained using a decoupling capacitorC_(DEC) that produces, with the preamplifier PA_(S), PA_(D) inputresistance, a first order high-pass filter whose corner frequency isgenerally lower than 20 Hz in order to avoid in-band audio signalperturbation.

More detailed representations of the differential preamplifier PA_(D)are shown in FIG. 3 (inverting configuration) and FIG. 4 (non-invertingconfiguration).

In the inverting case, due to noise generation, input resistors R_(1A)and R_(1B) cannot have high resistance values (typically from 10 kOhm to50 kOhm), whereas in the non-inverting solution resistors R_(3A) andR_(3B) are used only to bias the amplifiers OA inputs at a voltageV_(CM) midway between ground and the supply voltage. Accordingly,resistors R_(3A) and R_(3B) don't contribute in noise generation and canbe made with larger resistance values with respect to the inverting case(however, not more than some hundred of kOhms due to area occupation).In both cases, decoupling capacitors C_(DEC) of more than 100 nF areneeded and such large capacitance values would be difficult to integratein a chip. In fact, with actual technologies on chip integration of acapacitor having such large capacitance value would require an areagreater than 20 mm² and this fact made the integrating approachpractically unusable. US 2002/0125949 discloses the above problem of thewaste of area due to the integration in the chip of the decouplingcapacitor C_(DEC), confirming that the integration of the decouplingcapacitors C_(DEC) is practicable only for relatively reducedcapacitance values. Moreover, unfortunately, even with the PDImethodology the decoupling capacitors C_(DEC) can't be realized becauseof their high capacitance value and the fact that none of theirterminals are connected to a fixed potential. This is the reason why allthe existing known solutions use SMD capacitors. Since a preamplifierusually has several inputs (voice microphone, mono and stereo audiomicrophone, mono and stereo line-in, etc.) and each one could bedifferential, it is clear that on a mobile phone's PCB there are manySMD decoupling capacitors C_(DEC).

This is obviously a bottle neck for the area reduction strategy, andthere is a strong felt need of trying to find a solution to thisproblem, till now without success. The same above described problemholds for other consumer devices different from mobile phones, such asportable MP3 players, digital photo cameras, digital audio recorders,video cameras, and in general in devices with audio communication and/orrecording capabilities.

Resistance multipliers are already known in the state of art for examplefrom the publication “Mini Sixties Plus”, Joseph Kreutz, ELECTOR, vol.7/8, page 85 and from U.S. Pat. No. 5,652,537. However in the state ofart, it was never proposed or suggested to adopt these multipliers ininterface circuits for connecting a microphone circuit to a preamplifierwith the aim of making possible the integration of decoupling capacitorsC_(DEC). This is likely due to the fact that it is difficult to find asolution that adopts resistance multipliers such as the ones disclosedin the two above cited documents and which in the meantime is alsoadapted to provide a correct biasing, namely a fixed and stable biasing,of the preamplifier's input.

SUMMARY

In view of the above described limitations of the prior art interfacecircuits between a microphone circuit and a preamplifier, it is anobject of an embodiment to provide an interface circuit for connecting amicrophone circuit to a preamplifier which is adapted to solve the aboveindicated problem concerning the impossibility of reducing the areaoccupation below a desired value due to the presence of one or moredecoupling capacitors that cannot be integrated on a chip.

The above object is reached by an interface circuit adapted to connect amicrophone circuit to a preamplifier, the microphone circuit comprisinga microphone and at least an output node and the preamplifier comprisingat least an input node adapted to be connected to the output nodethrough the interface circuit. The interface circuit comprises at leasta decoupling capacitor for DC decoupling said input node from saidoutput node, the decoupling capacitor having a first endconnected/connectable to said input node and a second endconnected/connectable to said output node. The interface circuitcomprises at least one active circuit comprising a resistor with a firstend connected to the first end of the decoupling capacitor. Moreover,the interface circuit comprises a biasing circuit connected to a secondend of said resistor for biasing said input node of the preamplifierwith a desired bias voltage. The active circuit is adapted tooperatively act as a resistance multiplier and has an equivalentresistance that together with the decoupling capacitor defines ahigh-pass filter operatively connected/connectable between themicrophone and the preamplifier. Since the equivalent resistance can betheoretically made as high as desired, the decoupling capacitor can havea relatively reduced capacitance value, with respect to the abovedescribed prior art circuits, allowing the on-chip integration of thedecoupling capacitor. Moreover, thanks to the arrangement of the biasingcircuit it is possible to provide a fixed and stable bias voltage to thepreamplifier's input.

According to an embodiment, the active circuit comprises a unity gainamplifier circuit.

According to an embodiment, the active circuit comprises a first MOSsource follower and a second MOS source follower, each of said MOSsource followers having a respective gate terminal, a drain terminal anda source terminal, the gate terminal of the second MOS source followerbeing connected to the source terminal of the first MOS source follower.The first end of the resistor is connected to the gate terminal of thefirst MOS source follower and the second end of the resistor isconnected to the drain terminal of the first MOS source follower and tothe source terminal of the second MOS source follower.

According to a more particular embodiment:

-   -   the active circuit comprises a current generator having an        output terminal connected to the second end of the resistor and        having a control terminal; and    -   the biasing circuit comprises an operational amplifier having an        output terminal connected to said control terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomemore apparent from the following detailed description of exemplary butnon-limiting embodiments thereof, as illustrated in the attachedfigures, in which:

FIG. 1 shows a schematic view of a first example of prior art microphonesystem comprising a microphone circuit and a preamplifier;

FIG. 2 shows a schematic view of a second example of prior artmicrophone system comprising a microphone circuit and a preamplifier;

FIG. 3 shows a schematic view of a first example of a known preamplifierfor the microphone system of FIG. 2;

FIG. 4 shows a schematic view of a second example of known preamplifierfor the microphone system of FIG. 2;

FIG. 5 shows a very schematic view of a device provided with audiorecording and/or communication capabilities;

FIG. 6 shows a microphone preamplifier circuit comprising a preamplifierand an interface circuit provided with a biasing circuit for biasing thepreamplifier;

FIG. 7 shows a high level representation of the interface circuit ofFIG. 6;

FIG. 8 shows a partial view of the microphone preamplifier circuit ofFIG. 6, wherein a first embodiment of the biasing circuit is shown;

FIG. 9 shows a partial view of the microphone preamplifier circuit ofFIG. 6, wherein a second embodiment of the biasing circuit is shown; and

FIG. 10 shows a partial view of the microphone preamplifier circuit ofFIG. 6, wherein a third embodiment of the biasing circuit is shown.

DETAILED DESCRIPTION

In the attached figures identical or similar elements will be indicatedwith the same reference numbers/symbols.

FIG. 5 shows a very schematic view of an embodiment of mobile terminal5, such as for example a mobile phone 5, comprising a microphone circuitMC_(D). According to an embodiment, and without for this reasonintroducing any limitation, the microphone circuit MC_(D) may be similarto the differential microphone circuit MC_(D) represented in FIG. 2. Themicrophone circuit MC_(D) comprises a microphone 3, a microphone'sbiasing network and two differential output nodes M_(O), M_(O)′.According to an embodiment, the microphone's biasing network may beidentical or similar to the known biasing network of FIG. 2.

The mobile phone 5 comprises a circuit board 6 comprising an integratedmicrophone preamplifier circuit 60. According to the embodiment shown,the circuit board 6 further comprises an analog to digital converter 70and a digital audio processor 80.

As the general structure and the operation of a mobile terminal, such asfor example a mobile phone, are well known to a man skilled in thefield, for sake of conciseness they will not be detailed further in thefollowing part of the description. On the contrary, the following partof the description will be mainly focused on the microphone preamplifiercircuit 60. It is important to remark that such microphone preamplifiercircuit 60 can be also employed in systems and and/or devices differentfrom a mobile terminal 5, for example in general in devices having audiocommunication or recording capabilities such as, digital audiorecorders, MP3 players, photo-cameras etc.

FIG. 6 shows an embodiment of microphone preamplifier circuit 60comprising a preamplifier P_(A) and an interface circuit INT_(C) adaptedto connect the microphone 3, or more precisely the microphone circuitMC_(D), to the preamplifier P_(A).

According to the embodiment shown, the preamplifier P_(A) is adifferential preamplifier having two input nodes 10, 10′, twooperational amplifiers OA, three resistors R1, R_(2B), R_(2B). Theinterface circuit INT_(C) comprises two decoupling capacitors C_(DEC)and two active circuits UG_(AMP), UG_(AMP)′ each associated with acorresponding input node 10, 10′.

Since the structure of the interface circuit 60 in the embodiment shownis totally symmetrical, in the present description only the upper branchC_(DEC), UG_(AMP) of the interface circuit INT_(C) will be detailed,namely the branch comprised between the output node M_(O) and the inputnode 10, which is similar to the lower branch C_(DEC), UG_(AMP)′comprised between the output node M_(O)′ and the input node 10′.Moreover, it should be clear that even if in the present description aninterface circuit INT_(C) for connecting a differential microphonecircuit M_(CD) to a differential preamplifier P_(A) will be disclosed,the teachings of the present description can be easily extended by a manskilled in the field to the case in which the interface circuit isadapted to connect a single-ended microphone circuit to a single-endedpreamplifier.

With reference to FIG. 6, the decoupling capacitor C_(DEC) is providedfor DC decoupling the preamplifier's input node 10 from the output nodeM_(O), and has a first end connected/connectable to the input node 10and a second end connected/connectable to the output node M_(O).

The active circuit UG_(AMP) of the interface circuit INT_(C) isconnected to the first end of the decoupling capacitor C_(DEC) and isadapted to operatively act as a resistance multiplier, having anequivalent resistance that together with the capacitance of decouplingcapacitor C_(DEC) defines a high-pass filter operativelyconnected/connectable between the microphone circuit M_(CD) and thepreamplifier P_(A). According to an embodiment, the active circuitUG_(AMP) comprises a unity gain amplifier circuit. More particularly,according to the embodiment shown, the active circuit UG_(AMP)comprises:

-   -   a first MOS source follower M1 and a second MOS source follower        M2, each of said MOS source followers having a respective gate        terminal g1,g2, a drain terminal d1, d2 and a source terminal        s1, s2, the gate terminal g2 of the second MOS source follower        M2 being connected to the source terminal s1 of the first MOS        source follower M1;    -   a resistor R having a first end connected to the gate terminal        g1 of the first MOS source follower M1 and a second end        connected at a common node 20 to the drain terminal d1 of the        first MOS source follower M1 and to the source terminal s2 of        the second MOS source follower M2.

The input node 10 of the preamplifier P_(A) represents a common nodebetween the first end of the decoupling capacitor C_(DEC), the gateterminal g1 of the first MOS source follower M1 and the first end ofresistor R.

The active circuit UG_(AMP) comprises a biasing circuit B_Circ forbiasing the preamplifier P_(A) and in particular the input node 10 ofthe preamplifier P_(A) with a predetermined fixed and stable biasvoltage, for example to the common mode voltage V_(CM) of thepreamplifier P_(A). In the embodiment of FIG. 6 the biasing circuitB_Circ is connected to the common node 20, i.e. to the second end ofresistor R.

According to the embodiment shown, the active circuit UG_(AMP) furthercomprises a first current generator Ig1, having an output terminal d4connected at node 20 and having a control terminal g4 connected to thebias circuit B_Circ and adapted to bias the MOS source followers M1 andM2. In the particular example shown, the current generator Ig1 is a MOStransistor M4 and the control and output terminals of said generator Ig1are the gate g4 and the drain d4 terminals, respectively, of said MOStransistor M4.

Moreover, according to the embodiment shown, the active circuit UG_(AMP)further comprises a second current generator Ig2, in the examplecomprising a MOS transistor M3, for biasing the MOS source follower M1,having a gate terminal feed with a fixed voltage V_(B) and sourceterminal feed with a fixed voltage V_(a1) and a drain terminal connectedto the source terminal s1 of the MOS source follower M1.

A high level schematic representation of the active circuit UG_(AMP) ofFIG. 6 is shown in FIG. 7. The active circuit UG_(AMP) comprises a unitygain amplifier 90, more precisely an amplifier 90 having a gain A_(V)very close to 1, that produces the resistance multiplication, and abiasing circuit B_Circ adapted to fix the DC voltage of nodes 10 and 20to a desired bias voltage V_(CM). Thanks to the resistancemultiplication, the equivalent resistance R_(EQ) is:

$R_{EQ} = {R \cdot {\frac{1}{1 - A_{V}}.}}$

In the embodiment of FIG. 6, wherein the unity gain amplifier 90 isrealized using two MOS source followers M1 and M2 and whereinaccordingly the signal present on each terminal of the resistor R isabout the same, the equivalent resistance R_(EQ) is:

$R_{EQ} = {R \cdot \frac{1}{1 - A_{V\; 12}}}$where A_(V12) is the voltage gain between node 10 and node 20 and isequal to:

$A_{V\; 12} \cong \frac{{gm}\; 2}{{{gm}\; 2} + {1/Z_{2}}}$where:

-   -   gm2 is the transconductance of the MOS source follower M2,    -   Z₂ is the output resistance at node 20, equal to:

$\frac{1}{Z_{2}} \cong {{g\; d\; s\; 2} + {g\; d\; s\; 3} + {g\; d\; s\; 4} + {{\frac{{gm}\; 2}{{gm}\; 1} \cdot g}\; d\; s\; 3}}$where:

-   -   gm1 is the transconductance of the MOS source follower M1;    -   gds2 is the output conductance of MOS source follower M2;    -   gds3 is the output conductance of MOS transistor M3;    -   gds4 is the output conductance of MOS transistor M4.

It is clear from the present description that, since R_(EQ) can have avery high value (for example few hundred MOhms if the resistance valueof resistor R is about 100 kOhms), the decoupling capacitor C_(DEC) canhave a relatively reduced capacitance value (with respect to prior artinterface circuits), for example from 10 pf to 100 pf and can betherefore easily integrated on a chip.

With reference to FIGS. 8-10, three embodiments of possible biasingcircuits B_Circ that can be employed in the interface circuit INT_(C) ofFIG. 6 will be disclosed in the following part of the description. Suchcircuits B_Circ share among them the functions of forcing the biasingvoltage of node 20 to a desired bias value, in this specific exampleequal to the common mode voltage V_(CM) (for example half of the supplyvoltage) of the preamplifier P_(A), and of becoming of high impedancefor frequencies higher than a desired frequency (about 20 Hz forvoice/audio applications) in order to let node 20 follow the signal atnode 10, i.e. the input signal.

With reference to FIG. 8, according to a first embodiment, the biasingcircuit B_Circ comprises an operational amplifier OA_B having an outputterminal connected to the control terminal g4 of the first currentgenerator Ig1. The biasing circuit B_Circ further comprises a low passfilter D1, C5 comprising a reverse polarized diode D1 and a capacitor C5having a common node connected at first input of the operationalamplifier OA_B. The diode D1 is further connected to the common node 20of the active circuit UG_(AMP). The operational amplifier OA_B is anopen loop amplifier having a second input fed with the desired biasvoltage V_(CM). In the above biasing circuit B_Circ, the reversepolarized diode D1 is used to implement a high impedance (a P-N junctionat 0 Volts) of the low pass filter D1, C5.

For frequencies lower than the desired cut-off frequency (for example ofabout 20 Hz), the feedback of the operational amplifier OA_B is activeand such amplifier OA_B sets the node 20 at the desired bias voltageV_(CM). In the above embodiment of biasing circuit B_Circ of FIG. 7, theoperational amplifier OA_B is not provided with a direct feedbackbetween its input and its output but it is in an open-loopconfiguration. This works good if its gain is relatively low (<10)whereas for higher gain values there is a peaking in the frequencyresponse between the operational amplifier's input and node 20.

In a second embodiment of biasing circuit B_Circ, shown in FIG. 9, is itpossible to close the operational amplifier OA_B with a resistivefeedback R_(i), R_(f), thus improving the above explained embodiment ofFIG. 8. The resistive feedback R_(i), R_(f) is adapted for fixing thegain of the operational amplifier OA_B to a proper value (for example,not greater than 10), namely a value selected to avoid peaking in thefrequency response. In this case, due to the partition between thefeedback resistors R_(i), R_(f) a suitable voltage value V_(CM)* shallbe applied to the resistor R_(i) in order to apply to the operationalamplifier's input the desired bias voltage value V_(CM).

In a third embodiment of biasing circuit B_Circ, it is possible tofurther improve the above first and second embodiments. In particular,in the third embodiment shown in FIG. 10, the diode D1 has been replacedby a switched capacitor C7 and an RC low pass filter R9, C6 having arelatively small time constant with respect to the time constant τ ofthe low pass filter defined by switched capacitor C7 and capacitor C5.The switched capacitor C7 is equivalent to a resistor R7=1/f_(s)C7,where f_(s) is the clock frequency of the switched capacitor C7.Accordingly, it is possible to achieve a time constant equal to:τ=R7×C5=C5/(f _(s) ×C7).

The above time constant τ can be made as high as needed just making C7and f_(s) as small as possible. Moreover, such time constant τ isadvantageously insensitive to process spreads and temperature variationssince it depends only on a ratio between capacitances and a preciseclock frequency f_(s).

In the embodiment of FIG. 10, the resistor R9 and the capacitor C6 arenot strictly necessary for the correct operation of the biasing circuitB_Circ but their use is recommended in order to avoid possibledisturbances coming from the sampling structure and propagating to thesignal at node 20. Moreover, in the embodiment of FIG. 10, theoperational amplifier OA_B can have an open-loop or a closed-loopconfiguration depending on the same considerations described previouslywith reference to FIGS. 8 and 9. In the embodiment of FIG. 10 a clocksignal with frequency f_(s) is required, but this is not a problembecause a clock is usually present on chips adapted to process voice andaudio signals.

As is clear from the above description, thanks to the resistancemultiplication effect of the active circuit, the above describedembodiments of interface circuit have the advantage of requiring one ormore decoupling capacitors C_(DEC) having a relatively reduced valuesuch that said capacitors can be integrated in the preamplifier's P_(A)chip. The advantage in terms of area occupation is even greater when thepreamplifier P_(A) is shared between a plurality of n sources, such asfor example n microphones circuits. In this case it is possible toplace, or better to integrate, a multiplexer (for example, realized withCMOS transfer gates) just before the decoupling capacitor C_(DEC) inorder to allow the selective connection among the different n sourcesand the preamplifier P_(A). In this case, only two relatively smalldecoupling capacitors C_(DEC) (if the circuit is differential) arerequired instead of 2n external SMD capacitors, obtaining a largereduction in the area/space occupation.

Naturally, in order to satisfy contingent and specific requirements, aperson skilled in the art may apply to the above-described interfacecircuits many modifications and variations, all of which, however, areincluded within the scope of protection of the invention as defined bythe following claims.

The invention claimed is:
 1. An interface circuit adapted to connect amicrophone circuit to a preamplifier, wherein the microphone circuitcomprises a microphone and at least an output node and wherein thepreamplifier comprises at least an input node adapted to be connected tothe output node through the interface circuit, the interface circuitcomprises: at least a decoupling capacitor for DC decoupling the inputnode from the output node, the decoupling capacitor having a first endconnected, or configured to be connected, to the input node and a secondend connected or configured to be connected to the output node; at leastone active circuit comprising: a resistor with a first end connected tothe first end of the decoupling capacitor; a first MOS source follower;and a second MOS source follower, wherein each of the MOS sourcefollowers have a respective gate terminal, a drain terminal and a sourceterminal, and wherein the gate terminal of the second MOS sourcefollower is connected to the source terminal of the first MOS sourcefollower; and wherein the first end of the resistor is connected to thegate terminal of the first MOS source follower and the second end of theresistor is connected to the drain terminal of the first MOS sourcefollower and to the source terminal of the second MOS source follower;and a biasing circuit connected to a second end of the resistor forbiasing the input node of the preamplifier with a desired bias voltage;the interface circuit being configured to operatively act as aresistance multiplier and to have an equivalent resistance depending ona resistance of the resistor that together with the decoupling capacitordefines a high-pass filter operatively connected or configured to beconnected between the microphone and the preamplifier.
 2. The interfacecircuit according to claim 1, wherein the active circuit comprises aunity gain amplifier circuit.
 3. The interface circuit according toclaim 1, wherein: the active circuit further comprises a currentgenerator having an output terminal connected to the second end of theresistor and having a control terminal; and wherein the biasing circuitfurther comprises an operational amplifier having an output terminalconnected to the control terminal.
 4. The interface circuit according toclaim 3, wherein the current generator further comprises a MOStransistor comprising a gate terminal and a drain terminal, wherein thecontrol and output terminals of the current generator are the gate anddrain terminals of the MOS transistor respectively.
 5. The interfacecircuit according to claim 1, wherein the preamplifier is a preamplifierhaving a common mode voltage and wherein the biasing circuit is adaptedto bias the input node with the common mode voltage.
 6. The interfacecircuit according to claim 3, wherein the biasing circuit is a low passfilter comprising a reverse polarized diode and a capacitor having acommon node connected at a first input of the operational amplifier. 7.The interface circuit according to claim 6, wherein the operationalamplifier is an open loop operational amplifier having a second inputfed with the desired bias voltage.
 8. The interface circuit according toclaim 7, wherein the biasing circuit comprises a resistive feedbacknetwork of the operational amplifier.
 9. The interface circuit accordingto claim 3, wherein the biasing circuit comprises a first low passfilter comprising a switched capacitor and a second capacitor bothhaving a common node operatively connected or adapted to be connected ata first input of the operational amplifier, the operational amplifierhaving a second input fed with the desired bias voltage.
 10. Theinterface circuit according to claim 9, wherein the biasing circuitfurther comprises a second low pass filter of the RC type in series withthe first low pass filter, the second low pass filter having a timeconstant relatively small with respect a time constant of the first lowpass filter.
 11. The interface circuit according to claim 1, furthercomprising a multiplexer configured for selectively connecting thesecond end of the decoupling capacitor to one of a plurality ofmicrophone circuits, the multiplexer, the decoupling capacitor and thepreamplifier being integrated in a same chip.
 12. A microphonepreamplifier circuit comprising a preamplifier, an interface circuit anda microphone circuit; the microphone circuit comprises a microphone andat least an output node and the preamplifier comprises at least an inputnode adapted to be connected to the output node through the interfacecircuit, the interface circuit comprises: at least a decouplingcapacitor for DC decoupling the input node from the output node, thedecoupling capacitor having a first end connected, or configured to beconnected, to the input node and a second end connected or configured tobe connected to the output node; at least one active circuit comprising:a resistor with a first end connected to the first end of the decouplingcapacitor; a first MOS device and a second MOS device, each of the MOSdevices having a respective gate terminal, a drain terminal and a sourceterminal, the gate terminal of the second MOS device being connected tothe source terminal of the first MOS device, and wherein the first endof the resistor is connected to the gate terminal of the first MOSdevice and the second end of the resistor is connected to the drainterminal of the first MOS device and to the source terminal of thesecond MOS device; and a biasing circuit connected to a second end ofthe resistor for biasing the input node of the preamplifier with adesired bias voltage; the interface circuit being configured tooperatively act as a resistance multiplier and to have an equivalentresistance depending on a resistance of the resistor that together withthe decoupling capacitor defines a high-pass filter operativelyconnected or configured to be connected between the microphone and thepreamplifier.
 13. A device provided with audio recording capabilities,the device comprising a microphone system comprising at least onemicrophone circuit and a microphone preamplifier circuit; the microphonecircuit comprises a microphone and at least an output node; themicrophone preamplifier circuit comprises a preamplifier and aninterface circuit; the preamplifier comprises at least an input nodeadapted to be connected to the output node through the interfacecircuit, the interface circuit comprising: at least a decouplingcapacitor for DC decoupling the input node from the output node, thedecoupling capacitor having a first end connected, or configured to beconnected, to the input node and a second end connected or configured tobe connected to the output node; at least one active circuit comprising:a resistor with a first end connected to the first end of the decouplingcapacitor; a first MOS device and a second MOS device, each of the MOSdevices having a respective gate terminal, a drain terminal and a sourceterminal, the gate terminal of the second MOS device being connected tothe source terminal of the first MOS device, and wherein the first endof the resistor is connected to the gate terminal of the first MOSdevice and the second end of the resistor is connected to the drainterminal of the first MOS device and to the source terminal of thesecond MOS device; and a biasing circuit connected to a second end ofthe resistor for biasing the input node of the preamplifier with adesired bias voltage; the interface circuit being configured tooperatively act as a resistance multiplier and to have an equivalentresistance depending on a resistance of the resistor that together withthe decoupling capacitor defines a high-pass filter operativelyconnected or configured to be connected between the microphone and thepreamplifier.